1. Field of the Invention
This invention relates to a vital timer for energizing an output relay at the end of a preselected time interval. Also, the invention relates to my related inventions disclosed in U.S. Pat. Nos. 3,995,173, 4,090,173, 4,181,849 and 4,234,870, and my copending U.S. applications Ser. No. 157,658 filed June 9, 1980, now U.S. Pat. No. 4,368,534, Ser. No. 007,184 filed Jan. 29, 1979, now abandoned, and Ser. No. 119,655 filed Feb. 8, 1980, now U.S. Pat. No. 4,307,463 the disclosures of which are hereby incorporated by reference herein.
2. Description of the Prior Art
In the rail industry, it is often necessary to activate an output device a predetermined time interval after the occurrence of a particular event. For example, it may be desired to open the doors of a passenger car a predetermined time after the car has come to a stop. For this application, it is critically important that the output relay controlling the opening of the passenger car doors is not prematurely activated if the safety of the rail system is not to be compromised.
Aside from the application to the opening of the doors of a rail car, there are numerous other instances in which it is desired to activate an output device after the passage of a predetermined time period, and only after the time period has in fact expired. This is true from the electronic controls provided for rail switching and signaling, and virtually any application where safety is a prime consideration.
In the past, mechanical means have been used to perform the necessary timer function, and motor time element relays have long been used in the rail industry. While the mechanical timers have been suitable for many purposes, they exhibit relatively limited programmability and therefore have a relatively limited performance range. Furthermore, while the accuracy of the mechanical timers has been adequate for many applications, in other instances where high accuracy is a requirement, it is necessary to find alternate means for generating the time interval. Thus, as the rail industry in particular rushes into the electronic age, it is desirable to develop a reliable, safe and relatively inexpensive electronic replacement for the mechanical timer of the past.
Recently, attempts have been made abroad to apply computer techniques to fulfill the function of a vital timer. While the details are somewhat sketchy at this time, the general approach seems to be to utilize completely redundant mini-computers produced by different manufacturers and programmed by different programming teams to process redundantly the vital timer time interval and then activate an output device only in the event that the redundant mini-computer systems are in agreement as to the time of activation. The prevailing wisdom is that if you have different programming teams providing different programs for different computers, the likelihood of a common failure is slim and represents an acceptable risk. Nevertheless, since this technique of employing independently redundant mini-computer systems makes no provision for internal checking of the processing of either system, a fatal combination of failures is a distinct possibility. Furthermore, the independent redundancy concept necessarily entails considerable recurring and non-recurring costs to bring these systems to market, which represents a further compromise in the utility of that approach.